The invention relates to a gas-turbine engine combustor capable of burning both gas and liquid fuels and in particular, but not exclusively, a combustor operating under a lean-burn combustion process.
Lean-burn combustor designs, in which very little if any combustion air is introduced into the combustor downstream of the location of the burner air-fuel mixing arrangement, are currently prevalent. The great advantage of lean-burn systems is the reduction of the levels of harmful emissions under high engine-load conditions. A drawback, however, is the difficulty that is experienced in maintaining the integrity of the combustor flame during low-load conditions, so that xe2x80x9cflame-outxe2x80x9d, i.e., the simple extinction of the flame, does not occur.
To avoid flame-out at low engine-load conditions, prior-art designs have used techniques such as fuel-rich pilot-flame systems and staged fuel systems. The former are inclined to increase emission levels and the latter generally result in a complicated and expensive design.
The present invention aims to combine a reduction in harmful emissions with a reduction in complexity and consequently cost.
In its broadest aspect, the present invention provides a gas-turbine engine combustion system of the lean-burn type, having a combustor comprising a burner, a combustion pre-chamber and a combustion main chamber disposed in flow series, the burner comprising a burner head having a burner face including fuel injection means for the injection of fuel from the burner face into the pre-chamber, the combustor being arranged such that during operation of the combustor, a front face of a combustion flame burns closely adjacent the burner face, the burner further comprising fuel directing means for directing fuel towards the burner face during a first mode of operation of the combustor, and cooling air directing means for directing a flow of cooling air towards the burner face during a second mode of operation of the combustor.
According to a preferred embodiment of the present invention, a gas-turbine engine combustion system of the lean-burn type has a combustor comprising a burner, a combustion pre-chamber and a combustion main chamber disposed in flow series, the burner comprising
a burner head,
a burner face of the burner head, the burner face defining an upsteam extremity of the pre-chamber,
gas fuel injection means for the injection of gas-fuel from the burner head into the pre-chamber, and
liquid-fuel injection means separate from the gas-fuel injection means for the injection of liquid-fuel from the burner head into the pre-chamber,
the combustor being arranged such that during operation of the combustor a front face of a combustion flame burns closely adjacent a central part of the burner face, the combustion system further having
means for enabling changeover from gas fuel operation of the combustor to liquid fuel operation of the combustor, and
means operable during liquid-fuel operation of the combustor to prevent injection of gas fuel and enable injection of cooling air from the burner head into the prechamber,
the burner further comprising directing means, whereby gas-fuel is directed towards the central part of the burner face during gas-fuel operation of the combustor and cooling air is directed towards the central part of the burner face during liquid-fuel operation of the combustor.
It is convenient, but not essential, that the same directing means be utilized to direct both the gas fuel and the cooling air towards the central part of the burner face.
The gas-fuel injection means may include duct means adapted to inject the gas-fuel and the cooling air in an annular configuration towards the central part of the burner face.
The directing means may comprise lip means provided on the burner face and extending towards the central part of the burner face, the lip means being disposed relative to the injector means such as to deflect gas-fuel and air exiting the injector means towards the central part of the burner face.
The liquid-fuel injection means may be disposed between the gas-fuel injection means and the central part of the burner face. Preferably, the liquid-fuel injection means comprises a liquid-fuel duct means communicating with the burner face. An igniter may be disposed between the gas-fuel injection means and the liquid-fuel injection means, or between adjacent liquid-fuel injection means.
The liquid-fuel and gas-fuel injection means preferably comprise pilot gas-fuel injection means, pilot liquid-fuel injection means, main gas-fuel injection means and main liquid-fuel injection means, all the pilot and main fuel injection means being in communication with the burner face. Advantageously, the main liquid-fuel injection means is disposed radially outwards of the pilot gas-fuel injection means. The main gas-fuel injection means may be disposed radially outwards of the main liquid-fuel injection means.
The burner preferably includes a radial swirler disposed between the burner face and the pre-chamber, the swirler having a plurality of passages for the flow of combustion air through the swirler towards the central part of the burner face. Preferably, the main gas-fuel injection means communicates with at least one of the swirler passages adjacent a radially outer part of the passages, while the main liquid-fuel injection means communicates with at least one of the passages adjacent a radially inner part of the passages.
The combustion system includes fuel-inlet means communicating with the pilot and main gas-fuel and liquid-fuel injection means for the supply of fuel thereto, a control means being connected to the fuel-inlet means for controlling the flow of fuel into the pilot and main gas-fuel and liquid-fuel injection means such that during liquid-fuel operation, the control means diverts pilot gas-fuel away from the pilot gas-fuel injection means and connects to the latter a source of the cooling air.
The invention further provides a method of operating the above combustion system during a gas-fuel operation of the combustor, comprising the steps of:
initiating injection of pilot fuel and main fuel into the pre-chamber at predetermined respective mass flow rates, and
varying the respective mass flow rates of the injected pilot fuel and main fuel relative to a total gas-fuel mass flow rate between a start-up condition and a full-load condition of the engine, such that at the start-up condition of the engine, the total gas-fuel flow predominantly comprises pilot fuel and, at the full-load condition of the engine, the total gas-fuel flow predominantly comprises main fuel.
Preferably, at the start-up condition of the engine, the main gas-fuel provides not more than about 5% of total gas fuel flow, and the pilot gas-fuel provides not less than about 95% of total gas fuel flow, whereas at the full-load condition of the engine, the main gas-fuel provides not less than about 95% of total gas fuel flow, and the pilot gas-fuel provides not more than about 5% of total gas fuel flow, but more than 0% thereof.
The invention further provides a method of operating the above combustion system during a liquid-fuel operation of the combustor, comprising the steps of:
initiating injection of pilot liquid fuel into the pre-chamber at a predetermined mass flow rate during a start-up condition of the engine,
increasing the mass flow rate of pilot liquid fuel to increase engine power towards a full load condition of the engine,
initiating injection of main liquid fuel into the pre-chamber at a predetermined mass flow rate when a predetermined fraction of the full-load condition of the engine is attained,
continuously decreasing the supply of pilot fuel and increasing the supply of main fuel until the full-load condition of the engine is attained, and
injecting cooling air into the prechamber from the burner head using the directing means during said liquid-fuel operation of the combustor.
The above predetermined fraction of the full-load condition of the engine may be approximately 70% and at the full-load condition of the engine the main liquid fuel may provide not less than about 95% of total liquid fuel flow and the pilot liquid fuel may provide not more than about 5% of total liquid fuel flow, but more than 0% thereof.